The present invention relates generally to enhancing flow through a passage driven by differential pressure and more particularly, to enhancing moisture removal from pitot tubes or to enhancing boundary layer flow removal from total air temperature probes (TAT probes) in which there are practical limitations to passage quantity and size.
Pitot tubes are commonly used to measure stagnation pressure of a fluid. When used in combination with static pressure measurements, pitot tubes are used to determine dynamic pressure which is proportional to the velocity of the fluid. In order to conduct the stagnation pressure measurement, pitot tubes are typically formed from a long, generally cylindrical body that extends upstream into the fluid to isolate an upstream end of the pitot tube from flow disturbances. The upstream end includes an aperture communicating with an interior cavity of the pitot tube. Pressure transducers that communicate with the interior cavity are positioned to measure the stagnation pressure or, in other words, the static pressure of a fluid at a stagnation point (i.e. a point in which the fluid velocity equals zero).
In the context of an aircraft, pitot tubes measure the stagnation pressure of the aircraft moving through an airstream and, when used with static pressure measurements located elsewhere on the aircraft, are used to determine the air speed of the aircraft. Accuracy of the pitot tube measurement depends on maintaining an unobstructed cavity between the pitot tube inlet aperture and the pressure transducer location. However, aircraft experience a myriad of environmental conditions during flight including precipitation, moisture, and freezing temperatures less than −40° C. (−40° F.). As such, moisture and ice particles can accumulate within the pitot tube. To preserve the functionality of the pitot tube, heaters and drain holes are included to remove the moisture and ice particles from the pitot tube. The mass flow rate draining from the pitot tube is related to a differential pressure between the pitot tube cavity and the external static pressure as well as the drain hole diameter and length. In general, larger drain hole diameters permit more mass flow rate through the drain hole than smaller diameters.
TAT probes measure a fluid temperature at a stagnation point. Generally, TAT probes ingest fluid through an inlet and decrease the fluid velocity downstream from the inlet by passing the fluid through an expansion section. Once fluid velocity is reduced, a sensing element measures the temperature at a stagnation point within the probe. However, after the fluid enters the TAT probe, a boundary layer develops near interior walls of the probe. Like the pitot tube, TAT probes are heated to reduce icing within the probe. Heating the probe increases a temperature of the boundary layer flow near the walls. Measurement error is caused by boundary layer flow when it is allowed to interact with the sensing element. Therefore, TAT probes often include passages extending through the wall to remove the boundary layer flow. Like the pitot tube drain holes, flow through these passages is driven by a differential pressure. The inlet static pressure of the passage is generally greater than the outlet static pressure of the passage because fluid entering the TAT probe passes through an expansion, thereby increasing the static pressure of the fluid within the TAT probe.
However, some pitot tube and TAT probe applications have passages for which a maximum diameter is limited for practical reasons (e.g., the size of internal heating components, constraints limiting the quantity of passages). Therefore, pitot tubes and TAT probes require passages that have one or more features to improve the mass flow rate therethrough.